Post-transcriptional regulation of gene expression is critical for cell growth control and animal development. This project attacks SRPK1 and 2, two evolutionary conserved kinases that correspond to the major kinase activity for the SR family of splicing factors and regulators in mammalian cells. SR proteins are involved in many aspects of RNA metabolism, all of which appear to be regulated by phosphorylation. Strikingly, we recently found that mouse embyo fibroblasts genetically deleted of SRPK1 are transformed in the soft agar assay and in nude mice. Furthermore, SRPKs are localized in the cytoplasm and can be induced to translocate to the nucelus in response to specific signals. Thus, SRPKs are fundamentally important for cell growth control and the kinase system may be regulated by signaling. Building upon our expertise established in the previous funding periods of this project, long-term interest in post-transcriptional regulation of gene expression, and extensive published and unpublished findings, we propose to continue this project under three specific aims for the next phase of research. Aim 1 is to develop conditional knockout mice models to determine the functional requirement for both SRPK1 and 2 during mouse development and characterize the kinase knockout mouse embryo fibroblasts to provide genetic evidence that SRPKs regulate the function SR proteins in RNA metabolism in mammalian cells. Aim 2 is to address the putative tumor suppressor activity of SRPK1 by testing three specific hypotheses: (1) SRPK1 may regulate alternative splicing of oncogenes and tumor supressors via SR proteins;(2) SRPK1- mediated phosphorylation may act to reorganize newly exported mRNA-protein complex to regulate translation in the cytoplasm;and (3) SRPK1-mediated phosphorylation may play a critical role in maintaining genomic stability by modulating the coupling between transcription and splicing. Aim 3 is to understand how SRPKs might be regulated by signaling. A panel of SRPK-interacting proteins have been identified and many are known components of various signal transduction pathways. We design specific experiments to understand how some specific signals may be transduced via SRPKs to regulate gene expression at post- transcriptional levels. Together, the project will dissect an unprecedented cellular transformation pathway.